Abstract: We report our recent theoretical studies on the metal－to－semiconductor transition in squashed armchair single－walled carbon nanotubes. The transition can be achieved by a combined effect of the broken mirror symmetry and bond formation between the flattened faces in the squashed nanotubes. Furthermore, we reveal a general mechanism for this transition, namely the distinguishing of the two original equivalent sublattices in the nanotube (symmetry breaking) that opens an energy gap near the Fermi energy.